1. Technical Field
The invention relates to the deposition of films on substrates. More particularly, the invention relates to a method and apparatus for producing uniform, isotropic stresses in a sputtered film.
2. Description of the Prior Art
Thin films are often deposited on substrates by sputtering in a glow-discharge plasma, where ions accelerated out of the plasma knock atoms off of the target (source) material whence the atoms are transported to the substrate. A magnetically confined plasma generator (magnetron) is typically used to increase sputtering efficiency and to reduce the minimum operating pressure. Sputtering is a preferred deposition technique because it can be used for any material, because the energy of the depositing atoms helps film adherence, and because the substrates do not get very hot.
Uniformity of film thickness across large substrates is usually important, and one of two approaches is conventionally taken to achieve such uniformity.
One such approach is to position the substrates at a radius far from the target relative to substrate and target diameters. To increase throughput and use targets efficiently, many substrates are positioned at this radius over most of a hemisphere and are kept in a planetary (two-axis) motion so that they occupy a wide range of positions over the hemisphere during the course of the deposition time. This averages out deposition rate variation over the hemisphere.
The second approach uses a rectangular target that is larger than the substrate in the target's long dimension. The substrate is placed close to the target and is passed back and forth across it in linear transport so that the substrate is painted with a uniform swath of film in successive layers much like painting with a roller. Typically 100 nm of film are deposited in each pass.
Sputtering is used in the formation of various microelectronic structures. Among these structures is a patterned spring structure that is useful in such applications as device testing. For example, D. Smith and S. Alimonda, Photolithographically Patterned Spring Contact, U.S. Pat. No. 5,613,861 (25 Mar. 1997), U.S. Pat. No. 5,848,685 (15 Dec. 1998), and International Patent Application No. PCT/US 96/08018 (Filed 30 May 1996), disclose a photolithography patterned spring contact, which is “formed on a substrate and electrically connects contact pads on two devices. The spring contact also compensates for thermal and mechanical variations and other environmental factors. An inherent stress gradient in the spring contact causes a free portion of the spring to bend up and away from the substrate. An anchor portion remains fixed to the substrate and is electrically connected to a first contact pad on the substrate. The spring contact is made of an elastic material and the free portion compliantly contacts a second contact pad, thereby contacting the two contact pads.”
Such patterned spring technology depends on being able to control very high levels of film mechanical stress uniformly across a substrate. Stress is common in thin films and is usually undesirable. Indeed, many techniques of process control are used in planetary and linear-transport sputtering, as well as in other film-deposition processes, to minimize stress. Consequently, while many of the factors influencing stress are recognized, the state of the art is concerned with substantially eliminating such stresses.
Ion bombardment is known to increase compressive stress in any vacuum-deposition process. In magnetron sputtering, low plasma pressure increases compression, higher pressure creates tensile stress, and still higher pressure results in porous films that have no mechanical strength in the film plane. The magnetron sputter-deposition of films imparted with stress gradients by increasing plasma pressure during deposition is a presently preferred technique for implementing patterned spring technology.
Although it is known in the art how to minimize stress and how to produce high compressive or tensile stress, techniques for maximizing stress and of controlling uniform high stress across large substrates are not known. Both maximizing the stress level and making it uniform are desirable in connection with the fabrication of patterned spring structures. It would be advantageous to provide a method and apparatus for producing uniform, isotropic stresses in a sputtered film.